1. Field of the Invention
This invention relates to a distributed-feedback type semiconductor laser device and a method of producing the same.
2. Description of the Prior Art
In conventional distributed-feedback type semiconductor laser devices, a diffraction grating is formed all over the surface of a guide layer disposed adjacent an active region of the laser device which includes an active layer to provide for single longitudinal mode (SLM) operation. On the other hand, single transverse mode operation by a refractive index wave guide is achieved by the use of a structure formed externally at a position apart from the diffraction grating after all crystal growth is completed.
The foregoing known structure requires additional crystal growth and other manufacturing steps which give rise to problems that need to be solved. For example, a buried heterostructure (BH) type distributed-feedback semiconductor laser requires that crystal growth be effected three times. During the third crystal growth period, a crystal is grown in an inverted mesa form, which results in problems involving the boundary at which a leak current can occur if the boundary is badly formed, the current path, controllability of the p-n junction produced by diffusion of dopants when heat treatment is effected for a long time, and so on. On the other hand, in a ridge structure-type distributedfeedback semiconductor laser device, the refractive index wave guide characteristic depends largely on the thickness of an active region arranged at opposite sides of a ridge structure and on the ridge width. Therefore, the production of the ridge structure must be precisely controlled.
It has been proposed in Japanese Laid-open Patented Publication No. 59-80984 to provide a surface illuminant distributed-feedback type semiconductor laser device in which a periodic uneven surface, that is, a diffraction grating, is formed on the surface of a light guiding layer. Then, such surface of the light guiding layer other than a striped-shaped region extending in the forwarding direction of the laser light, is etched to a depth short of the underlying active layer with the object of obtaining plane regions at the etched surface portion, whereupon, a clad layer is grown on the light guiding layer. However, since the desired plane regions are obtained by etching surface portions on which the diffraction grating had been earlier formed, wave-shaped wrinkles inevitably remain in the nominally plane regions. The clad layer cannot be satisfactorily grown on the wrinkled plane regions of the guiding layer, and this adversely influences the useful life of the laser device and the stability of its operations.
Persons having an obligation to assign their inventions to the assignee of the present application have proposed, in Japanese Patent Application No. 61-141137, a distributed-feedback type semiconductor laser device in which an active layer is interposed between a pair of clad layers and a stripe-shaped distributed feedback means constituted by a diffraction grating is formed in a guide layer on the active layer, while surface portions of the guide layer at opposite sides of the stripe-shape diffraction grating are maintained substantially planar, that is, wrinkle free. Since the guide layer surface portions at opposite sides of the stripe-shaped diffraction grating are planar, each crystal layer grown successively on the guide layer after the diffraction grating is formed can be provided with good crystallinity for prolonging the useful life of the semiconductor laser device and improving the stability of its operations. However, if the diffraction grating is formed only on a narrow region of the guide layer, as in the foregoing proposal, it is difficult to control the manufacturing conditions so as to ensure the uniform formation of the diffraction grating with good reproducibility.